Dielectric substrate with selectively controlled effective permittivity and loss tangent

ABSTRACT

A substrate ( 300 ) for an RF device includes a plurality of layers ( 102 ) of dielectric material cofired in a stack. The plurality of layers ( 102 ) is formed from a material having a permittivity. Selected ones of the layers ( 102 ) have a pattern of perforations ( 106 ) formed in at least one perforated area ( 104 ). The perforated areas ( 104 ) are generally aligned with one another in the stack to lower one or more of an effective value of a permittivity and a loss tangent in a least one spatially defined region ( 504 ) of the substrate ( 300 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/465,074filed Jun. 19, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. F005521 between the Defense Advance Research ProjectsAgency, the United States Naval Research Laboratory and HarrisCorporation.

BACKGROUND OF THE INVENTION

1. Statement of the Technical Field

The invention concerns dielectric substrates for RF circuits, and moreparticularly dielectric substrates with effective permittivity valuesthat can be independently controlled in predetermined portions of thesubstrate.

2. Description of the Related Art

The design and fabrication of microwave circuits and antennas are basedon standard materials that are available for printed wiring boards orceramic substrates. Improvements in the standard materials areincremental and tend to be infrequent. Attempts at modifying theproperties of the substrates by various means have been attemptedoccasionally, but they have not generally resulted in any process thatis practical, reliable and robust.

U.S. Pat. No. 5,559,055 discloses a system for reducing the interlayerdielectric constant in a multilayer interconnect structure to increasedevice speed and performance. More particularly, the RC time constant ofa semiconductor device is reduced by decreasing the capacitance C. Thedecrease in capacitance is achieved by replacing the interlayer silicondioxide (dielectric constant of 4.0) with air (dielectric constant of1.0). In either case, the final effective dielectric constant of thedevice is lowered, which results in higher device speed.

U.S. Pat. No. 6,175,337 discloses a high-gain, dielectric loaded,slotted waveguide antenna. The antenna makes use of a tailoreddielectric structure in which the effective dielectric constant isincrementally or continuously reduced to have a dielectric constantclose to that of the free-space value at an outer surface a distancefrom the waveguide array. The tailoring of the effective dielectricconstant is achieved by layering a given number of slabs of differentdielectric constants with sequentially reduced values, or by varying thechemical composition of the material, or by varying the density of thematerial imbedded with high dielectric constant particles.

Another approach to controlling the effective permittivity of adielectric substrate is to perforate the board material in selectedareas. This approach could be particularly well suited to ceramicsubstrates as they tend have a relatively high loss tangent and aretherefore lossy. However, the perforating technique has also sufferedfrom certain drawbacks. For example, the perforation of the substratehas tended to produce a weakened mechanical structure, particularly whenthe percentage of substrate material removed is high. Also, theperforations in the substrate are open to the environment and cantherefore allow contaminants to collect within the structure. Theconventional perforation techniques have also tended to producedielectric substrates with effective permittivity values that are notconsistent at each measurable point on the surface.

Another disadvantage of conventional perforated substrate system is thatsimply perforating the substrate will produce openings on both sides ofthe board. This interferes with the RF circuitry disposed on thesubstrate. Perforations can be drilled only partially through thesubstrate material to leave a continuous surface on at least one side.For example, laser drilling can be used for this purpose. However,difficulties are encountered in controlling the accuracy of the laserdrilling process. In particular, it is difficult to precisely controlthe depth of drilled perforations so as to maintain a stable value ofpermittivity and loss tangent across the surface of the perforated area.Moreover, the drilling process leaves the internal structure of thesubstrate exposed on at least one side of the board.

SUMMARY OF THE INVENTION

The invention concerns a method for fabricating a substrate for an RFdevice. The method includes the steps of forming a pattern ofperforations in a plurality of layers of a dielectric material in atleast one perforated area of each layer and arranging the plurality oflayers in a stack. At least one perforated area in each of the pluralityof layers is at least partially aligned with another perforated area ofanother layer in the stack to lower an effective value of permittivityand an effective value of a loss tangent in a least one spatiallydefined region of the substrate, as compared to a bulk value ofpermittivity and loss tangent for the dielectric material. The layerscan be formed from a low temperature cofired ceramic tape or a hightemperature cofired ceramic tape. The method can also include the stepof firing the stack and forming an RF circuit component on the substratein the spatially defined region. The pattern of perforations can beexcluded from one or more outer layers of the substrate to seal theinner perforations from dust and contamination.

According to one aspect of the invention, the perforation pattern foreach of the plurality of layers can be selected so that the effectivevalue of permittivity is substantially the same at each measurable pointof the spatially defined region of the substrate. Further, the patterncan be varied among the plurality of layers to ensure structuralrigidity and a final substrate with a planar outer surface. For example,the pattern can be offset from layer to layer.

According to another aspect of the invention, the method can alsoinclude selecting the pattern of perforations in the plurality ofperforated areas to produce different values for the effectivepermittivity in a plurality of the spatially defined regions on thesubstrate. Alternatively, the pattern of perforations in each of thelayers can be selected so as to cause the effective permittivity toselectively vary in a predetermined manner across a surface of thesubstrate defined by the spatially defined region.

The invention also concerns a substrate for an RF device. The substrateis comprised of a plurality of layers of dielectric material cofired ina stack. Each of the plurality of layers is formed from a materialhaving a permittivity and having a pattern of perforations formed in atleast one perforated area. The perforated areas of each respective oneof the layers is advantageously aligned at least partially withassociated perforated areas in adjacent layers to lower an effectivevalue of a permittivity and an effective loss tangent in one or morespatially defined regions of the substrate. The substrate layers can becomprised of a high temperature cofired ceramic tape or a lowtemperature cofired ceramic tape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a single layer of a dielectric material that canbe used to construct a substrate with selectively controlled effectivepermittivity.

FIG. 2 is a cross-sectional view of the single layer of dielectricmaterial taken along line 2—2 in FIG. 1.

FIG. 3 is a drawing that is useful for understanding the assembly of asubstrate.

FIG. 4 is a perspective view of the completed substrate.

FIG. 5 is a cross-sectional view of the completed substrate taken alongline 5—5 in FIG. 4.

FIG. 6A is an enlarged cross-sectional view of a portion of thecompleted substrate in the area 6—6 in FIG. 5 showing verticallystaggered perforations.

FIG. 6B is an enlarged cross-sectional view of a portion of thecompleted substrate in the area 6—6 in FIG. 5 showing vertically alignedperforations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1–3, a process is illustrated for manufacturing alaminated substrate formed from a plurality of layers 102 having definedareas with selectively controlled values of permittivity and losstangent. FIG. 1 is a top view of a layer 102 formed of a dielectricmaterial that can be used to construct the substrate 300 shown in FIG.3. A cross-sectional view of the single layer 102, taken along line 2—2,is shown in FIG. 2.

The layer 102 is preferably formed from an unfired ceramic tape.According to a preferred embodiment, the layer 102 can be comprised of alow or high temperature type cofired glass/ceramic tape. Glass/ceramictapes such as those described herein are well known in the art. Forexample, low temperature 951 cofire Green Tape™ can be used for thispurpose. Green Tape™ is Au and Ag compatible, has acceptable mechanicalproperties with regard to thermal coefficient of expansion (TCE) andrelative strength. It is available in thicknesses ranging from 114 μm to254 μm and is designed for use as an insulating layer in hybridcircuits, multichip modules, single chip packages, and ceramic printedwire boards, including RF circuit boards. Low temperature 951 cofireGreen Tape™ is available from The DuPont Company's MicrocircuitMaterials division which is located at 14 T. W. Alexander Drive,Research Triangle Park, N.C.

As used herein, the reference to low temperature cofired ceramics refersto ceramics that can be fired at relatively low temperatures. Forexample, firing temperatures for such material range are typically lessthan about 850 degrees Centigrade. By comparison, high temperatureceramics are typically fired at temperatures in excess of 1500 degreesCentigrade.

A typical electronic circuit module can be made with Green Tape bycutting tape foil to size, punching vias and filling same with thickfilm conductive paste. Subsequently, the conductive interconnect linesare patterned on the tape. The operation is repeated multiple times ifadditional layers are required. After all of the individual layers arecomplete, they can be collated, stacked, and laminated. At this pointthe stacked layers are commonly referred to as a “green” multilayer.Using standard processing techniques, the multilayer is fired and cut tosize. Finally, the top layer is completed by forming resistors, and goldand silver bearing conductors.

Although low and high temperature cofired glass/ceramic substrates arepreferred for use in the present invention, it should be noted thatother dielectric substrate layers can also be used and the invention isnot intended to be limited in this regard. A preferred thickness of thelayers 102 is presently between about 100 μm to 300 μm, but it should beunderstood that layers that are thinner or thicker can also be used.

According to a preferred embodiment of the invention illustrated inFIGS. 1 and 2, each layer 102 can have one or more perforated areas 104which are formed with a pattern of very small perforations 106. At leastone perforated area in each of the layers 102 is preferably at leastpartially aligned with at least one other perforated area of anotherlayer 102 in the stack. This will lower an effective value of apermittivity and an effective value of a loss tangent in a spatiallydefined region of the substrate coincident with the aligned portion ofthe perforated areas as compared to a bulk value of permittivity andloss tangent for the dielectric material. Perforated areas areconsidered to be at least partially aligned when at least a portion of aperforated area 104 for a layer 102 overlaps at least a portion of asecond perforated area of another layer in the stack.

The perforations 106 are preferably formed in each layer 102 while thelayer is still in the pre-fired state. The pattern, size, shape andspacing of the perforations 106 are selected to adjust the effectivepermittivity and effective loss tangent in the perforated areas. Theshape of the perforations is not critical to the invention. However,square perforations are presently preferred relative to circularperforations because of the larger amount of material that can beeffectively removed while still maintaining structural integrity of thelayer. If the perforations 106 have a square outline, they can be in therange of about 0.004 inches to 0.2 inches on each side, depending on thewavelength of the RF signals of interest in a particular application. Ingeneral, the size of the perforations is preferably no larger than about1/10λ to 1/50λ, where λ is equal to one wavelength at the frequency ofinterest. The relative size selected will be dependent somewhat on thefrequency of interest and fabrication limits. For example, at lowerfrequencies and/or with improved fabrication techniques, perforationsless than 1/50λ are possible. In fact, the concept works for any sizescale below the previously mention 1/10λ limit and can be realized evendown to the nanoscale level with the same bulk properties.

The perforations 106 can be formed by punching or drilling each layerseparately using commercially available equipment. According to apreferred embodiment, commercially available precision computercontrolled high speed punching equipment can be used for this purpose.For example, the perforations can be formed using an MP 4150 typeautomated punch available from Unichem Industries, Inc. of San Clemente,Calif. Computer controlled high speed punching equipment is preferredfor this process because the pattern of perforations 106 in perforatedarea 104 can vary somewhat from layer to layer. For example, thearrangement of perforations, their spacing and shape may be varied.Alternatively, the perforations can be vertically aligned or can besimply offset slightly from one layer to the next while maintainingessentially the same pattern.

It is possible for the perforation pattern in layers 102 to be variedfrom layer to layer or they may be aligned directly with theperforations in the layer above and below without affecting theplanarity of the top or bottom surface. Structural rigidity is retainedby applying either approach to arranging the perforated layers.

The effective permittivity of the perforated area generally decreaseslinearly as more substrate is removed. Significantly, however, it hasbeen found that the effective loss tangent for the perforated area 104will decrease rapidly, and in a non-linear fashion, as the percentage ofmaterial comprising layer 102 that is removed ranges from about 60% to90%. The optimal volume of substrate removed will depend upon a varietyof factors. For example, consideration must be given to the increasingfragility of the structure as larger amounts of material are removed.Also, care must be taken so that the perforated area 104 does not createa non-planar outermost surface 114 when the layers 102 are arranged in astack.

Referring again to FIG. 3, at least one outermost dielectric layer 108can be formed from the same material as the layers 102 and added to thestack of layers 102 as shown. However, the outer dielectric layer 108 ispreferably devoid of perforated areas 104. In this way, the outerdielectric layer 108 can act as a sealing layer to prevent the intrusionof dust, moisture and other contaminants into the perforations 106 inthe layers 102. Depending upon the electrical and mechanicalrequirements for a particular application, it can be desirable toinclude multiple dielectric layers 108. The additional layers canprovide increased rigidity and mechanical strength as may be requireddepending upon the intended use and environment (such as shock andvibration). Once all of the dielectric layers have been arranged in astack, they can be fired in a manner consistent with the requirements ofthe particular type of layer material.

After firing, the stack of layers 102, 108 can be arranged on a base112. According to a preferred embodiment, base 112 is a conductive sheetor foil. For example copper sheet can be used for this purpose. However,it should be understood that the invention is not so limited and a rigiddielectric or semiconductor material can also be used to form base 112.An adhesive layer 110 is preferably provided between the layers 102 andthe base 112 to secure the stack of dielectric layers to the base 112.Adhesive layer 110 is preferably a conductive adhesive. For example anelectronic grade conductive film adhesive can be used for this purpose.Such adhesive is typically a silver filled epoxy with 70% silverparticles. Adhesives of this type are commercially available and can becured at relatively low temperatures. For example, typical curing timesare about 125 degrees centigrade for about 2 hours in a low temperaturecure oven. Curing time will vary depending on the particular adhesivematerial that is selected.

Once the cured stack of layers 102, 108 have been placed on the adhesive110, they are preferably maintained in a stationary position until theadhesive has cured. Sliding or moving the perforated layers 102 cancause the conductive adhesive to be inadvertently forced up into theperforations 106, thereby negatively affecting the electricalperformance of the substrate 300. According to an alternativeembodiment, one or more solid dielectric layers 109 made from the samematerial as layers 102 but without any perforations can optionally beinterposed between the lowermost layer 102 and adhesive layer 110. Thesolid dielectric layer 109 can be used to prevent the unwanted intrusionof the adhesive layer 110 into the perforations 106. Also, such layerscan be desirable for improved mechanical properties as may be necessarydepending upon the intended use and environmental conditions.

FIG. 4 is a cross-sectional view of a completed substrate 300 withselectively controlled permittivity. FIG. 5 is a cross-sectional viewthrough substrate 300 taken along line 5—5. The substrate 300 includesone or more spatially defined regions 504 that have a lower effectivevalue of permittivity and a lower effective value of loss tangent, ascompared to a bulk value of permittivity and loss tangent for thedielectric material comprising the layers 102, 108. The lower values aredue to the selective removal of dielectric material as shown.

FIG. 6 a is an enlarged view of a portion of FIG. 5 defined by line 6—6showing an offset perforation pattern from one layer 102 to the nextlayer 102 as previously described. Notably, this can result invertically staggered perforations as shown. However, the invention isnot limited in this regard, and it is also possible to form perforatedareas that make use of a consistent pattern from one layer to the nextso as to produce vertically aligned perforations as illustrated in FIG.6B. In yet another embodiment, the patterns can vary from layer to layerwhile remaining within the perforated area of each layer 102.

Conductive elements 116 can be screen printed on outermost layer 108 onan area of outermost layer 108 extending over spatially defined region504. The screen printing on the array is typically an electronics gradeconductive epoxy or ink that cures in the 100 degree to 125 degreerange. The conductive elements 116 can comprise any of a wide variety ofRF elements that have an electrical characteristic modified as a resultof the modified permittivity and/or loss tangent of the spatiallydefined region 504. For example, and without limitation, conductiveelements 116 can be antenna elements associated with an array, filterelements, transmission line elements, transformer elements, stubs, andso on.

The foregoing process offers great flexibility for RF designers withoutrequiring costly changes to conventional processing methods. Having madethe fundamental choice of the dielectric thickness, selected portions ofthe substrate can now be specifically tailored to achieve the desireddielectric properties. The flexibility of this approach gives the RFdesigner almost unlimited control over effective permittivity andeffective loss tangent without changing processing steps.

According to one aspect of the invention, the perforation pattern foreach of the plurality of layers can be selected so that the effectivevalue of permittivity is substantially the same at each measurable pointof the spatially defined region of the substrate. Further, amultiplicity of spatially defined regions 504 can be defined within thesubstrate, each with either the same or different effective values ofpermittivity and loss tangent. Alternatively, the pattern ofperforations in each of the layers 102 can be selected so as to causethe effective permittivity and loss tangent to selectively vary in apredetermined manner across the surface 114 of the substrate defined bythe spatially defined region. This can include varying the perforationsize, perforation shape and/or perforation spacing within the perforatedarea 104 of one or more layers 102. For example, progressively more orless dielectric material can be removed from one or more layers 102along a particular direction defined along the surface 114 so as tocause the permittivity and loss tangent to decrease or increase in apredetermined manner. In any case, the perforation pattern can be variedamong the plurality of layers to ensure structural rigidity and a finalsubstrate 300 with a planar outer surface 114.

1. A substrate for an RF device, comprising: a plurality of layers ofdielectric material cofired in a stack, each of said plurality of layersformed from a material having a permittivity and having a pattern ofperforations formed in at least one perforated area; and said perforatedareas of each respective one of said plurality of layers at leastpartially aligned with one another in said stack to lower at least oneof an effective value of a permittivity and a loss tangent in at leastone spatially defined region of said substrate, wherein said effectivepermittivity varies in a predetermined manner across a surface of saidsubstrate defined by said spatially defined region in accordance with avariation of said pattern of said perforations.
 2. The substrateaccording to claim 1 wherein said plurality of layers are comprised of aceramic tape.
 3. The substrate according to claim 1 wherein saidplurality of layers are comprised of a low temperature type cofiredceramic tape.
 4. The substrate according to claim 1 wherein saidperforations of each said perforated area are aligned with one another.5. The substrate according to claim 1 wherein said pattern ofperforations is excluded from at least one outermost layer of saidsubstrate.
 6. A substrate for an RF device, comprising: a plurality oflayers of dielectric material cofired in a stack, each of said pluralityof layers formed from a material having a permittivity and having apattern of perforations formed in at least one perforated area; and saidperforated areas of each respective one of said plurality of layers atleast partially aligned with one another in said stack to lower at leastone of an effective value of a permittivity and a loss tangent in atleast one spatially defined region of said substrate, wherein saidpattern of perforations for each of said plurality of layers is arrangedso that said effective value of permittivity is substantially the sameat each measurable point of said spatially defined region of saidsubstrate.
 7. The substrate according to claim 1 wherein said pattern ofperforations is varied among said plurality of layers.
 8. The substrateaccording to claim 1 wherein said pattern of perforations is offset fromlayer to layer.
 9. The substrate according to claim 1 wherein saidpattern of perforations is formed in a plurality of said perforatedareas of each said layer to produce a plurality of said spatiallydefined regions, each having at least one of an effective value ofpermittivity and a loss tangent lower than a permittivity and losstangent for said dielectric material.
 10. The substrate according toclaim 9 wherein said pattern of perforations in said plurality ofperforated areas produces different values for at least one of saideffective permittivity and said loss tangent in at least a first one ofsaid spatially defined regions as compared to at least a second one ofsaid spatially defined regions.
 11. A substrate for an RF device,comprising: a plurality of layers of dielectric material cofired in astack, each of said plurality of layers formed from a material having apermittivity and having a pattern of perforations formed in at least oneperforated area; and said perforated areas of each respective one ofsaid plurality of layers at least partially aligned with one another insaid stack to lower at least one of an effective value of a permittivityand a loss tangent in at least one spatially defined region of saidsubstrate, wherein said pattern of perforations in each of said layerscauses said effective permittivity to selectively vary in apredetermined manner across a surface of said substrate defined by saidspatially defined region.